Date:   Sat, 14 Sep 2019 00:26:01 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     David Hildenbrand <david@...hat.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal@...nel.org" <sashal@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: RE: [PATCH] hv_balloon: Add the support of hibernation

> From: David Hildenbrand <david@...hat.com>
> Sent: Friday, September 13, 2019 2:44 PM
>
> > On recent Windows Server 2019+ hosts, the toolstacks on the hosts
> > guarantees that Dynamic Memory and Memory Resizing can not be enabled
> > if the virtual ACPI S4 state is enabled, and vice versa. Please refer to the
> > long write-up I made here.
>
> Hah, so the patch here is not actually relevant for modern Hyper-V

Correct.

> installations. (I would have loved to read that in the patch description
> - but maybe I missed that)

I'll add the related description into the changelog of v2 of this patch.

> > And, to make the hibernation functionality automated, the host is able to
> > send a "please hibernate" message to the VM via the Hyper-V shutdown
> > device upon the user's request (e.g. via GUI or scripting): see [...]
> > When the host sends the message,
> > it checks if the virtual ACPI S4 state is enabled for the VM: if not, the host
> > refuses to send the message. This means that the user does want to make
> > sure the virtual ACPI S4 state is enabled for the VM, if the user of the VM
> > wants to use the hibernation feature, and this means Dynamic Memory
> > and Memory Resizing can not be active due to the restrictions from the
> > host toolstack.
>
> Okay, *but* this is a current limitation. Just saying. If you could at
> least support balloon inflate/deflate, that would be a clear win for
> users. And less configuration knobs.

For Hyper-V (on recently hosts), Dynamic Memory (and Memory Resizing)
and hibernation are mutually exclusive and as I mentioned the host toolstack
guarantees they can not be both enabled. This is a host limitation and the VM
(i.e. we the Linux team) can do nothing about this. Note: here "enable
hibernation for a VM" means "enable the virtual ACPI S4 state for the VM".

By default a VM running on Hyper-V doesn't have the S4 state enabled, and
balloon inflate/deflate are indeed supported.

The knob (I think you mean the virtual ACPI S4 state) is introduced in the
host side design of the VM hibernation feature, and is enforced in the
host toolstack (as I described about the host-to-VM "please hibernate"
message). No knob or module parameter is introduced by the VM here.

> > And the hibernation functionality won't be officially supported on old
> > Windows Server hosts.
> >
> > So, IMHO we can't be bother to implement the idea you described in
> > detail. Sorry. :-)
>
> No worries, I neither develop for, use or work with Hyper-V. I was just
> reading along and wondering why you basically make the hv_balloon
> unusable in these environments. (initially I thought, "why don't you
> just disallow probing the device completely")

The Hyper-V team told me that: when hibernation is enabled & used for
a VM the only purpose of loading hv_balloon is that the driver can
still report the VM's memory pressure to the host, and it looks due to
some (non-technical?) reason the Hyper-V team thinks this info can be
useful.

> I am aware of the (hypervisor) issues of hibernation/suspend when it
> comes to balloon drivers / memory hot(un)plug. (currently working on
> virtio-mem myself and initially decided to block any
> hibernation/suspension attempts in case the driver is loaded and memory
> was plugged/unplugged)
>
> >
> > And, while I agree your idea is good, technically speaking I suspect it may
> > not be really useful, because once hv_balloon allows balloon-up/down,
> > hv_balloon effectively loses control of memory pages: after the host
> > takes some memory away, the VM never knows when exactly the
> > host will give it back -- actually the host never guarantees how soon
> > it will give the memory back. Consequently, the VM almost immediately
> > ends up in an un-hibernatable state...
> If you go via the host, you might be able to make sure to request to
> deflate the balloon before you try to hibernate, and inflate again when
> back up. You might even ask the user for permissions. Of course, once
> you deflated the balloon, it might not be guaranteed to inflate the
> balloon to the original size. But after all, it's "dynamic memory", so
> it might even be what the name suggests. It could be very well
> controlled from the host.
>
> If you go via the guest, you would first have to tell your hypervisor
> "please allow me to deflate so I can hibernate", or something like that.
> After hibernation (or some time X), the host might then decide to
> inflate again.
>
> E.g., take a look at virtio-balloon. When suspending, it simply deflates
> (without asking ...), to inflate again when resuming. Not saying that's
> the best approach (it's not :) ), but one approach to at least make it work.

Yes, I noticed this a few months ago. I think a major difference in Hyper-V
ballooning mechanism is that: all the deflate/inflate requests are from
the host and the VM can never proactively ask the host to deflate/inflate
the VM's memory. All that the VM can do is report its memory pressure
to the host and hope the host will soon give back the memory that was
taken away by the host.

I personally like the approach used in virtio-balloon. :-)

> Anyhow, just some comments from my side :) I can see how Windows Server
> worked around that issue right now by just XOR'ing both features.
>
> David / dhildenb

Thanks for sharing your thoughts!

Thanks,
-- Dexuan

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